Kim Jong-un and his uncle Jang Song-thaek fought over the nation’s lucrative shellfish exports, culminating in a firefight between their forces in recent months, a report said, providing a rare glimpse of deep political rifts and how power games are played in the North Korean regime.

Kim had ordered his 67-year-old uncle to give up control of lucrative businesses, especially seafood farms previously run by the military, but the elder refused, according to a report by The New York Times on Tuesday.

The crabs, clams and coal, among other goods, were in recent years exported mainly to China, a country with whom Jang had deep ties.

The businesses had been controlled by the military, to sustain its troops and operations, before Kim handed over some of the trading rights to his cabinet shortly after he took power, the newspaper said.

The profits then passed through Jang’s hands or through state agencies he controlled, it said.

However, when Kim saw that troops were underfed, he ordered Jang to return the businesses to the military, but was refused, the report said. Kim apparently sent more than 100 troops to the seafood farms, sparking a battle with Jang’s loyalists. The troops were “badly” defeated.

The firefight, which “enraged" Kim Jong-un, was said to have taken place either in September or early October, the report said, citing intelligence from South Korean and American officials and accounts from lawyers close to the matter.

The clash, showing Jang’s open defiance of Kim, was “the last straw”, it said, and eventually led to his trial and execution.

Jang, who was married to Kim Jong-il's only sister, was executed on December 12 on charges that included plotting a coup and corruption.

The execution, the biggest political upheaval since Kim took power two years ago, sparked speculation that Jang had lost out in a power struggle with hardline army generals.

Jang's wife reportedly survived the purge, remaining on the list of party officials.

About 88 per cent of North Korea's trade last year involved China, according to figures from the South's Korea Trade-Investment Promotion Agency. Exports to China were worth US$2.4 billion last year.